Passive (not active) transport of drugs across biological membranes is influenced by protein binding. Binding may occur - with plasma proteins or - with non-specific tissue proteins in addition to the drug’s receptors. ***Only drug that is not bound to proteins (i.e., free or unbound drug) can diffuse across membranes not only affects the activity of the drug (bound = inactive) But also can influence its distribution from one compartment to another. This is particularly true with respect to glomerular filtration and passive transport. Free + Protein  Bound Drug Drug Extensive plasma protein binding will cause more drug to stay in the central blood compartment. Therefore drugs which bind strongly to plasma protein tend to have lower volumes of distribution. these plasma proteins, albumin, which comprises 50 % of the total proteins binds the widest range of drugs. Acidic drugs commonly bind to albumin, while basic drugs often bind to alpha1-acid glycoproteins and lipoproteins. Many endogenous substances, steroids, vitamins, and metal ions are bound to globulins. When drugs bind to protein, Albumin concentration is reduced The exchange of proteins between plasma and interstitial compartment (normally proceeds at a rate of 5% plasma protein per our) will be hampered. The diffusion of plasma the to interstitial fluid is increased by: Inflammatory process Pregnancy use of oral contraceptives Diabetes Septic shock Pulmonary Edema

1. PROTEIN BINDING
1
ROHIT BHARTI
M. PHARM. 1 ST YR.
CHANNABASWESHWAR PHARMACY COLLEGE,
LATUR.

2. Protein Binding
Passive (not active) transport of drugs across biological
membranes is influenced by protein binding.
Binding may occur
- with plasma proteins or
- with non-specific tissue proteins in addition to the
drug’s receptors.
***Only drug that is not bound to proteins (i.e., free or
unbound drug) can diffuse across membranes
2

3. Protein binding
not only affects the activity of the drug (bound = inactive)
But also can influence its distribution from one compartment
to another.
This is particularly true with respect to glomerular filtration
and passive transport.
Free + Protein  Bound
Drug Drug
3

4. 4

5. Plasma Extracellular water
Plasma protein Tissue protein
drug
5

6. Plasma Proteins bindings
6

7. The fraction of total drug in plasma that is
bound is determined by
 the drug concentration,
 its affinity for the binding sites, and
 the number of binding sites.
7

8. Binding of drug to proteins may:
8
 Facilitate the distribution of drugs
 Inactivate the drug by not enabling a sufficient
concentration of free drug to develop at a receptor
site
 Retard the excretion of a drug

9. Extensive plasma protein binding will cause more drug to
stay in the central blood compartment. Therefore drugs
which bind strongly to plasma protein tend to have lower
volumes of distribution.
these plasma proteins, albumin, which comprises 50
% of the total proteins binds the widest range of
drugs.
Acidic drugs commonly bind to albumin, while
basic drugs often bind to alpha1-acid glycoproteins
and lipoproteins.
Many endogenous substances, steroids, vitamins,
and metal ions are bound to globulins.
9

10. HUMAN SERUM ALBUMIN
10
 Is the most important protein that binds to drug
molecule due to its high concentration compared
with other proteins ( mol.wt. 65,000 )
 It binds both acidic and basic
 Constitute 5% of the total plasma
 Both endogenous compounds such as fatty acids,
bilirubin & tryptophan as well as drugs bind to HSA

11. Albumin
 Expansion of vascular
compartment
 Increased catabolism
 Decreased synthesis
 Hemorrhage / exudative
losses,
 Renal/gut losses
 Increased capillary
permeability
 Decreased lymphatic return
Serum
albumin
concentrations
• Infusion of plasma or
albumin
• Decreased catabolism
• Increased synthesis
(e.g. insulin)
• Contraction of vascular
compartment (e.g.
vasoconstrictors)
11

12. Albumin
12
 Has 2 main binding sites with high constant of
association (104-106 M-1)
 The sites are charged positively and bind mainly
anionic molecules
 Ex: NSAIDs
 Carboxylic acids bind to site II (hydrophylic forces)
 Non-carboxylic acid (enol derivative): site I
 Oxicam : both sites (the warfarine site)

13. NSAID’s, sulphonamides, phenytoin
Benzodiazepines, ibuprofen,
cloxacillin, fatty acids medium chain
13

14. ∂1-ACIDGLYCOPROTEIN
14
 Also known as orosomucoid (∂1-globulin)
(mol.wt. 44,000)
 Binds to numerous drugs
 Have greater affinity for basic than acidic drugs molecules
 Binds only basic and highly lipophilic drugs
 It binds to a drugs like imipramine, amitriptyline, lidoacine,
propranolol, quinidine etc.

15. LIPOPROTEINS
15
 Plasma conc. Is much less than HAS, AAG.
 It binds to lipophilic drugs because of their high
lipid content.
 Mol wt. varies from 2 lakhs to 34 lakhs depending
on their chemical composition.
 Binding of drugs to lipoproteins is non-competitive
because of no specific and non specific binding
sites.

16. CLASSIFICATION OF LIPOPROTEINS
1. Chylomicrons (least dense & largest in size)
2. Very low density liporoteins ( VLDL )
3. Low density lipoproteins ( LDL )
4. High density lipoproteins
The main physiological role of lipoproteins is circulation
of lipids to tissues through the blood.
Also play an important role in the transport of drugs to
tissues.
16

17. Drug binding to lipoproteins
 Non-restrictive
17
 Involves lipid solubilization of the drug into the
lipidic core of the lipoproteins and/or
interaction with the surface phospholipids
 Because of multiple binding possibilities drug
such as imipramine and propanolol show a
relatively constant free fraction in plasma

18. 18

19. Things to remember:
19
 Many drugs bind to the same receptor site but drugs
with higher affinity will replace those drugs with
lower affinity by competition
 Only free and unbound drugs exert therapeutic effect
by interacting with receptors

20. Drugs may bind to protein through:
 Hydrophobic Interaction
 Proposed by Kauzmann
20
 tendency to develop of hydrophobic molecules or parts of
molecules to avoid water because they are not readily
accommodated in the H-bond structure of water

21. Drugs may bind to protein through:
 Self-Association
21
 Some drug may self dissociate to form dimers, trimers or
aggregates of larger size
 Dimers or trimers - is a reaction product of two or three identical
molecules
 May affect solubility, diffusion, transport, therapeutic action of
drugs

22. Protein binding is determined by:
 Dialysis
 Ultracentrifugation
 Ultrafiltration
 Sephadex-gel filtration
 Molecular filtration
 Electrophoresis
 Agar plate test
22

23. The Pharmacokinetic Importance
of Protein Binding
23
 Drug-protein binding influences the distribution
equilibrium of the drug
 Plasma proteins exert a buffer and transport
function in the distribution process
 Only free and unbound drug acts can leave the
circulatory system and diffuse into the tissue

24. Disease and Protein Binding
 Protein binding will be affected by the presence of diseases
Drugs showing Decrease Extent of Protein Binding
in the following diseases:
LIVER RENAL
Dapsone
Diazepam
Morphine
Phenytoin
Prednisolone
Quinidine
Tolbutamide
Triamterene
Barbiturates Salicylates
Cardiac Glycosides Sulfonamides
Chlordiazepoxide Triamterene
Clofibrate
Diazepam
Diazoxide
Furosemide
Morphine
Phenylbutazone
Phenytoin 24

25. Disease and Protein Binding
 When drugs bind to protein, Albumin concentration is
reduced
 The exchange of proteins between plasma and interstitial
compartment (normally proceeds at a rate of 5% plasma
protein per our) will be hampered.
 The diffusion of plasma the to interstitial fluid is
increased by:
 Inflammatory process
 Pregnancy
 use of oral contraceptives
 Diabetes
 Septic shock
 Pulmonary Edema
25

26. Is there often displacement of drug from the
binding site?
No
26
 For a substantial displacement to take place, the
displacer must occupy most of the available
binding site thereby lowering the binding site
available to the primary drug

27. Drug plasma protein binding
27
 Expressed in % or by fu (free fraction)
>90% = highly bound

28. The free fraction : fu
Definition:
28
free concentration
total concentration
Cfree
Ctot
 fu = =

29. Ctot is a function of Cfree , not the inverse
Distribution
Action
29
Measured total concentration
F F
Bmax
Kd Alb.
Elimination Interaction

30. 30
The free, the Bound
&
the total concentration

31. 31

B C
max free
K C
C
D free
bound


The bound concentration
Cbound
Cfree
Bmax
KD
• The bound concentration
Bmax : maximal concentration of
Bmax/2
binding sites
– proportionnal to plasma protein concentration
KD : free drug concentration corresponding to half maximal binding
– inversely proportional to drug affinity for the protein binding

32. Ctot is a function of Cfree
Ctot = Cfree + Cbind
Ctot = Cfree + Bmax x Cfree
Kd + Cfree
Dependent variable Parameters Independent variable
controlled by Clfree
32

33. 33
Relationship between fu, the Free and the
bound concentrations

34. Effect of modifications of plasma protein
concentration (Bmax) on fu
bindin
g- 34
D
K
max D
u
B K
f


Plasma protein increase
Bmax is increased
fu is increased
fu is decreased
Plasma protein decrease
Bmax is decreased

35. Algorithm for determining clinical significance of
potential binding displacement interaction
no
no
no
low
bindin
g- 35
Is drug of interest >90% protein
bound?
Yes
Does the drug have a narrow
therapeutic index ?
Yes
What is the hepatic extraction ratio of
the drug ?
High
Is the drug given IV?
Clinically significant
interaction not likely
no
Would a transient increase
in free drug concentration
be clinically relevant ?
Clinically significant interaction likely.
Perform a clinical study to quantify effects
Yes
Yes